The present invention relates to apparatus and methods for monitoring one or more parameters of the blood of a living organism.
Certain constituents in the blood affect the absorption of light at various wavelengths by the blood. For example, oxygen in the blood binds to hemoglobin to form oxyhemoglobin. Oxyhemoglobin absorbs light more strongly in the infrared region than in the red region, whereas hemoglobin exhibits the reverse behavior. Therefore, highly oxygenated blood with a high concentration of oxyhemoglobin and a low concentration of hemoglobin will tend to have a high ratio of optical transmissivity in the red region to optical transmissivity in the infrared region. The ratio of transmissivities of the blood at red and infrared wavelengths can be employed as a measure of oxygen saturation.
This principle has been used heretofore in oximeters for monitoring oxygen saturation of the blood in the body of a living organism, as, for example, in patients undergoing surgery. As disclosed in U.S. Pat. No. 4,407,290, oximeters for this purpose may include red light and infrared light emitting diodes together with a photodetector. The diodes and photodetector typically are incorporated in a probe arranged to fit on a body structure such as an earlobe or a fingertip, so that light from the diodes is transmitted through the body structure to the photodetector. The infrared and red light emitting diodes are switched on and off in alternating sequence at a switching frequency far greater than the pulse frequency. The signal produced by the photodetector includes alternating portions representing red and infrared light passing through the body structure. These alternating portions are segregated by sampling devices operating in synchronism with the red/infrared switching, so as to provide separate signals on separate channels representing the red and infrared light transmission of the body structure. After amplification and low-pass filtering to remove signal components at or above the switching frequency, each of the separate signals represents a plot of optical transmissivity of the body structure at a particular wavelength versus time.
Because the volume of blood in the body structure varies with the pulsatile flow of blood in the body, each such signal includes an AC component caused only by optical absorption by the blood and varying at the pulse frequency or heart rate of the organism. Each such signal also includes an invariant or DC component related to other absorption, such as absorption by tissues other than blood in the body structure. According to well known mathematical formulae, set forth in said U.S. Pat. No. 4,407,290, the oxygen saturation in the blood can be derived from the magnitudes of the AC and DC components of these signals.
As also set forth in the '290 patent, the same general arrangement can be employed to monitor constituents of the blood other than oxygen such as carbon dioxide, carbon monoxide (as carboxyhemoglobin) and/or blood glucose, provided that the other constituents have some effect on the optical properties of the blood.
Measurement apparatus and methods of this type have been widely adopted in the medical profession. However, such apparatus and methods have been subject to interference from ambient light falling on the photodetector. The signal processing devices used to recover the AC and DC components after amplification of the photodetector signal have been provided with circuits for cancelling signal components caused by ambient light. Generally, these circuits operate by obtaining a "dark current" signal representing the amplified photodetector signal during intervals when both of the light emitting diodes are off and hence all of the light falling on the photodetector represents ambient light. The dark current signal value can be used to cancel the ambient light component in signals representing infrared and red light.
This approach provides only a partial solution to the ambient light interference problem. The dark current cancellation circuitry adds complexity and cost to the apparatus. Moreover, the ambient light ordinarily flickers at about twice the local power line frequency (100 or 120 Hz), thus introducing a substantial component at these frequencies into the photodetector signal. The low-pass filters must be arranged to suppress these flicker components while passing the AC component at the pulse frequency and also maintaining acceptable limits on phase distortion of the filterd signals. The low-pass filters therefore may require expensive hand-matched components to achieve proper performance.
Moreover, the ambient light signals may saturate the initial or front end amplifier connected to the photodetector. Thus, The signals caused by ambient light may cause the front end amplifier to exceed its maximum rated output, resulting in unpredictable fluctuations in the amplifier output. To prevent saturation of the front end amplifier, its gain may be limited, but this in turn requires higher gain in subsequent stages, more amplification stages or both. Baffles have been used to reduce the amount of ambient light reaching the photodetector and thus prevent saturation. These baffles add further complexity and cost, and are only partially effective.
In addition, interference from sources other than ambient light can saturate the front end amplifier or create spurious signals. In particular, electromagnetic fields from electrosurgical devices or the like may create substantial signals in the photodetector or its leads through capacitive or inductive coupling. The shielding used to protect the photodetector and leads from such interference adds further cost, complexity and bulk.
Accordingly, there have been significant unmet needs heretofore for still further improvements in blood constituent monitoring apparatus such as medical oximeters.